Believing we only have ourselves, that we are separate from the world, becomes self-fulfilling.
Believing that we are not separate, that we are but a part of the whole, is also self-fulfilling.
Because all beliefs are self-fulfilling.
If you feel alone, you will occupy yourself with your needs and desires, and your own happiness.
If you feel inseparable from the world, you will occupy yourself with matters beyond your own needs and desires.
If you are wrong about being separate you risk losing your only opportunity to be happy.
If you are wrong about being inseparable you have still managed to make the world a better place by spreading happiness to others.
And of course, the happiness returns to you amplified by the contributions of countless others.
Think about it.